1. Technical Field
The present invention relates to a load controller, and more particularly to a load controller that PWM-controls a load.
2. Background Art
In vehicles, some of devices are required to operate in accordance with a drive instruction by the input of a control signal from an input switch even in a state that an ignition switch is turned off. In such devices, when the input switch is turned off, a current (dark current) needs to be lowered as much as possible, preferably to be set to zero from the viewpoint of a protection of a battery.
Thus, when a device is to be realized in which a PWM (Pulse Width Modulation) output operation is carried out by inputting a control signal (fixed input) under a state that an ignition switch is turned off, for instance, a load controller is found that is disclosed in JP-A-2001-148294. In this load controller, a load is PWM-controlled by a drive control signal whose frequency and duty ratio are set to a prescribed frequency and a prescribed duty ratio in accordance with a chopping wave generated by a chopping wave generating circuit.
Further, in a load controller disclosed in JP-A-2004-248093, current cut-off switching elements are added to all control parts for carrying out a drive control of load driving switching elements.
Further, in the load controller, when the dark current is zero during turning off an input, the accuracy of an operating threshold value voltage of an input part is deteriorated so that a malfunction is liable to occur due to noise or the like. Thus, as the input part of the load controller, such a structure as an input processing circuit disclosed in JP-A-2007-159059 is considered to be adopted. In this input processing circuit, a comparator having a hysteresis applied as input and output characteristics is arranged and a Schmitt circuit is connected to the input. The threshold value of the Schmitt circuit is set to be lower than that of the comparator to control the operation of the comparator by the output of the Schmitt circuit and remove a dark current in a stand-by state. Further, the hysteresis is applied to the comparator as the input and output characteristics to obtain a structure strong to noise. This structure is effective to a malfunction due to a leakage resistance component generated when an input switch is splashed with water or the like.
In a device disclosed in JP-A-2001-345684, a circuit example is shown that can reduce a dark current even when a gate leakage current of a MOSFET for driving a load is increased.
However, in the load controller disclosed in JP-A-2001-148294, even when the control input is turned off, an inner circuit always operates. Thus, a problem arises that a large amount of dark current is supplied.
Further, in the load controller disclosed in JP-A-2004-248093, when the current cut-off switching elements are added to all the control parts in a circuit, a dark current can be suppressed substantially to zero, however, a problem arises that the scale of the circuit is enlarged to enlarge the device.
Further, when the input processing circuit disclosed in JP-A-2007-159059 tries to be applied to the load controller, a problem arises that the dark current is suppressed only in the input part, but is supplied to other parts. Further, another problem arises that elements forming the circuit such as an inverter for enabling the Schmitt circuit or the comparator are increased to enlarge the scale of the circuit.
Further, in the load controller disclosed in JP-A-2001-345684, a charge pump is necessary so that the scale of the circuit is enlarged.